Hardness gets measured on different machines — Brinell (HB), Rockwell C (HRC, for harder steel), and Rockwell B (HRB, for softer) — and a test report might use any of them. This chart lines them up, plus the approximate tensile strength that goes with each, so you can read across from whatever number you were handed.
Steel only. These conversions are for non-austenitic steels (carbon and alloy). They do not hold for stainless, aluminum, brass, or cast iron — converting those this way gives misleading numbers. And every conversion is approximate; an actual test on the part is always more reliable.
| Brinell (HB) | Rockwell C (HRC) | Rockwell B (HRB) | Tensile (ksi, approx) |
|---|---|---|---|
| 601 | 57.3 | — | 309 |
| 555 | 54.7 | — | 285 |
| 514 | 52.1 | — | 263 |
| 477 | 49.6 | — | 243 |
| 444 | 47.1 | — | 225 |
| 415 | 44.5 | — | 210 |
| 388 | 41.8 | — | 195 |
| 363 | 39.1 | — | 182 |
| 341 | 36.6 | — | 170 |
| 321 | 34.3 | — | 160 |
| 302 | 32.1 | — | 150 |
| 285 | 29.9 | — | 141 |
| 269 | 27.6 | — | 133 |
| 255 | 25.4 | — | 126 |
| 241 | 22.8 | 100.0 | 118 |
| 229 | 20.5 | 98.2 | 111 |
| 217 | — | 96.4 | 105 |
| 207 | — | 94.6 | 100 |
| 197 | — | 92.8 | 95 |
| 187 | — | 90.7 | 90 |
| 179 | — | 89.0 | 87 |
| 170 | — | 86.8 | 83 |
| 156 | — | 82.9 | 76 |
| 149 | — | 80.8 | 73 |
It maps onto bolt grades nicely: a Grade 2 bolt (~74 ksi) sits near 150 HB; Grade 5 (~120 ksi) around 250 HB / 24 HRC; Grade 8 (~150 ksi) around 300 HB / 32 HRC; and an alloy socket cap screw (~180 ksi) up near 360 HB / 39 HRC. Full strength numbers are on the bolt grades & strength chart.
Why it’s only approximate: the relationship between hardness and strength varies a little with the steel’s makeup, so treat these as close estimates — good for cross-reading a report or sanity-checking a number, not for certifying a part.
